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International Journal of Contemporary Practices - Vol. 1, Issue. 6 ISSN: 2231-5608

RF-MEMS Devices: Problems Regarding Reliability And Degradation

Mechanisms

Tejinder Pal SinghResearch Scholar, Dept. of Physics, Shri Jagdishparsad Jabharmal Tiberwala University, Jhunjhunu, Rajsthan, India

Brijender KahanwalResearch Scholar, Dept. of Computer Sc. & Engg., Shri Jagdishparsad Jabharmal Tiberwala University, Jhunjhunu,

Rajsthan, India

Dr. R. K. ChoudharyDean (Academics, Director, RPET Group of Institutions, Bastara, Karnal, Haryana, India

ABSTRACT

Now-a-days, the MEMS technology employed for radio-frequency or microwave

applications is continually developing rapidly. This RF-MEMS technology is creating and

facing various problems related to reliability and degradation mechanics. In this paper,

we will present an overview of the most significant failure /degradation mechanisms

and reliability issues related to RF-MEMS devices. Although, our knowledge concerning

failure mechanisms and reliability problems is still very incomplete, yet knowledge about

this area is increasing day by day. This paper discusses reliability issues related to

fabrication, metallic contact, electrostatic actuation and packaging.

Key Words: - RF-MEMS, stiction, reliability, electrostatic actuation, packaging.

1. INTRODUCTION

RF-MEMS devices are actually the micro-electro-mechanical-systems (MEMS) used for

RF (radio frequency) and microwave applications. In the last ten years, RF-MEMS

technology has become most promising and emerging technology. Recently,

researchers are providing more attention towards this technology because of its skills

in implementing reconfigurable passive networks for the coming generation multi-

standards and multi-frequency wireless communication systems [1].

The main emphasis of the researchers now is on a particular class of RF-MEMS

devices that includes varacters, capacitive switches, ohmic contact based switches andmultiplexers. All these devices operate at RF or microwave frequencies. The major

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The reliability of RF-MEMS devices is determined by understanding the root cause of

all concerned degradation modes using a rigorous physics-based approach. One must

expose degradation modes by applying drive factors (like overvoltage) or

environmental factors (like humidity and temperature) which could accelerate

degradation. After knowing the degradation modes, we can perform the experiments

to find the acceleration factors and can estimate the lifetime of a well-designed and

packaged device. A significant strategy is existing design, also mentioned as design-

for-reliability, as explained in reference [4]. The process is very iterative. Once a

degradation mechanism is disclosed, the design, fabrication, packaging etc can then be

improved to minimize or eliminate the degradation mode. For some high volume

markets and safety-critical applications, reliability has been extensively studied and

has resulted parts with degradation rates at the ppm level of lifetimes of over ten

years [6]. For small volume markets where reliability is very critical (telecom),

extremely reliable RF-MEMS devices have also been demonstrated.

Depending upon the fabrication materials and environmental stress conditions, these

devices are subjected to diverse failure modes. A list of general failure mechanisms of

RF-MEMS devices is given in table 1 [4], [5]. Many degradation modes listed here can

be eliminated through suitable design and packaging.

Table 1: General RF-MEMS degradation mechanisms:

Degradation Mechanisms Accelerated Factors / Causes

Fracture Overload Fracture Fatigue Fracture

Creep/Plastic Deformation

Applied Stress Thermal Stress Intrinsic Stress

Stiction Capillary Forces Van der Waals molecular Forces Electrostatic Forces Solid Bridging

Wear Adhesive Abrasive Corrosive

Degradation of Dielectrics Charging

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Leakage Breakdown

Electro migration Current density Temperature

Delamination Thermal shock Mechanical shock

Surface Contamination Absorption Oxidation

Pitting in Surface Number of Cycles Electrostatic Discharge

3. Reliability classes of RF-MEMS:The classification of RF-MEMS devices is recently becomes a hot issue in such a way

that it has tendency to include any device which is made with at least one step of

micro-machining technology. So, it has become necessary to make a division of

various RF-MEMS devices in such a way that it becomes significant for studies

regarding reliability. This is important to make some common criteria for the

accelerated tests and ageing models. Three different classes of reliability of RF-MEMS

devices are briefed in the table 2. This classification of RF-MEMS devices has been

done in accordance with the level of mechanical complexity and boundary conditions

[2], [7].

The class-I has all the passive components that have been designed for diminished

losses through micro-machining fabrication. Mechanical movements of any part of the

structure of this class of RF-MEMS devices are not required during the functioning

and working. However, some deformations might take place during various processes

involved in fabrication. Reliability and stability of this class of RF-MEMS devices in the

long term do not alter significantly from those of conventional RF passive

components. Stability problems of the structures of these devices might take place to

thin dielectric membranes. Often, these dielectric membranes are used for high

quality factor passive components fabricated by using micro-machining. When heat

diffusion of the bulk material is not good, then the membranes also have tendency toexpose thermal problems. In addition to these problems, the devices which are under

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repeated temperature cycles during assembly and packaging process can be prone to

structural deformations that cause failure of the device.

Table 2: Classification of RF-MEMS Devices [2].

Class I II III

Micro-machined

StructuresYes Yes Yes

Movable Parts No Yes Yes

Impact No No Yes

Examples of RF-

MEMS Devices

High-Q Suspended

Inductors: spiral, self-

assembled coils; low-

loss RF-Membranes;

RF-CMOS substrate

removal post-

processing

Very High-Q micro-

electro-mechanical

resonators;

continuously tuning

capacitors.

Ohmic contacts

RFMEMS relays;

switched capacitors;

capacitive coupling

RF-MEMS switches

and multiplexers.

The second class of RF-MEMS devices demands mechanical movement of some part

during the working of the devices. This class of RF-MEMS devices consists of devices

having micro-machined structure and moveable parts. Notable examples of this class

are very high quality factor micro-electro- mechanical resonators and continuously

tuning capacitors. Due to repeated mechanical movements and vibrations, novel

stress mechanisms are introduced on the constituted parts of these devices. Plastic

deformations, mechanical relaxations, fatigue, creep etc can disturb the stability of

electro-mechanical behavior of these devices. All these failure and degradation

mechanisms cause the mechanical failure of second class of RF-MEMS devices. In

addition to this, oxidation and absorption like surface effects can cause stresses in

moving and oscillating part. As a result complex stability problems are introduced

that help in the failure of device.

The third class of RF-MEMS devices comprises of all the devices demanding two

distinguished mechanical moveable parts to attain and keep contact during a definite

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time of cycle of the operation. Novel problems related to reliability are caused due to

the presence of mechanical contact between the moving parts of device. These

reliability problems may be of mechanical type and electrical type. The major effect

that diminishes the working of devices is the stiction of mechanical parts that keep the

mechanical contact. Due to stiction of mechanical parts restoration of resting position

becomes almost impossible even after the removal of actuation force. The stiction can

happen due to many factors like redistribution and accumulation of electric charge in

dielectric slabs, capillary effects due to humid environment, micro welding of metals

due DC or RF power etc.

Examples of this class of devices are ohmic-contact RF- MEMS relays, switchedcapacitor, capacitive coupling RF-MEMS switches and multiplexers .Electrical ohmic

contacts between two metallic surfaces may be affected from stability problems that

arise due to number of cycles, variation in resistance of ohmic contacts, transfer and

erosion of material, surface contaminations and other surface effects like absorption

and oxidation.

4. Reliability Problems of RF-MEMS Devices

There exist a large numbers of possible reliability problems and failure mechanism

happening in RF-MEMS devices. These failure mechanism and reliability problems

paint a very complex scenario for the life time testing of these devices. This demands

novel methodologies, accelerated tests and new degradation models of RF-MEMS

devices. The diverse reliability problems of RF-MEMS devices can be related to their

fabrication process, related to electrostatic actuation, metallic contact and packaging.

(A) Reliability Problems Related to fabrication: -- Poor functioning of RF-MEMS can

appear directly after the manufacture or after a relatively short lifetime. This poor

functioning of devices is typically to fabrication related issues. Two types of problems

relating to fabrication can take place one mechanical and the other is electrostatic

type. The presence of residual tension within the structural material of the device, or

bad tension slopes, if not taken into consideration within the models during the

fabrication design, may cause permanent deformations of the released structures. All

this will lead to failure of the device or may result in bad performances at least. These

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problems can be identified by non-invasive inspection techniques like optical

interferometry quickly and easily.

The reliability problems associated with electrostatic fabrication affects badly the

dielectric layers which are used for isolation in capacitive switches. The electrostatic

reliability problems are also due to accumulation of charge during different steps

involved in fabrication process. As a result, there occurs a shift in actuation from the

original design value. This shift leads to failure of device because it is not according to

electro-mechanical specifications of the RF-MEMS device.

(B) Reliability Problems of Contact Material: The main considerations in designing the

ohmic contact are the contact area and adherence force. The reliability and stability of

direct metal-to-metal ohmic contact during the working of the RF-MEMS devices can

be diminished by many degradation mechanisms like electro-migration, micro-

welding of metals due to DC or RF power, softening of metal, transfer of material,

erosion of material, surface contaminations and other surface effects occurring due to

oxidation and absorption. Both the number of cycles and the total time spent by the

switch in the actuated state are critical factors for these degrading mechanisms.

Lifetime of the ohmic contact in these devices is also defined by the hot and cold

switching requirements.

(C) Reliability Problems Related to Electrostatic Actuation: Dielectric materials have

tendency to be affected by accumulation of steady and slowly moving charges. The

charges may be due to different mechanisms like dielectric polarization,

contamination of surface due to oxidation and absorption, defects in the crystalline

structures of the dielectrics. This charge distribution with the isolation layer will

cause a shift in effective electrostatic force at a given applied voltage. There occur two

different effects, according to literature, which depend upon the accumulation of net

charge within the dielectric.

One possibility is when net charge accumulates within the dielectric either during

fabrication process or during the lifetime cycling due to applied voltages. In this case,

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a shift in the d-V and C-V curves is observed [8]. Both the pull-in and pull-out voltages

will get changed. As a result, the device will fail to actuate at a required voltage.

The second possibility arises due to the non-uniform distribution of charge across the

geometry of device, while keeping the charge zero in the dielectric layer. Rottenberg

explained how the variations in charge can be responsible for failure of device. The

failure took place due to disappearance of the pull-out bias [9]. Non-uniform

distribution of the charge may occur due to fabrication processing issues and from

non-uniform field distribution during the device actuation and due to the residual air-

gaps present in the position of down-state.

(D) Packaging Related Reliability Problems: -- Reliability issues of various RF-MEMS

packaged devices are application dependent; hence, there is not a common set of

reliability problems. From the knowledge of failure mechanisms of a system, we come

to understand the reliability of that system. The main degradation mechanism of the

RF-MEMS device is stiction. In stiction, microscopic adhesion takes place when two

surfaces come in contact with each other. Before the integration of contact metal RF-

MEMS devices into communication systems becomes a reality, stiction problem needs

to be resolved. Many researchers have proposed to reduce stiction by either selecting

contact materials with less adhesion [10], applying chemical surface treatment [11] or

by eliminating contamination with plasma cleaning or by a mechanical approach to

provide enough restoring force to overcome the adhesion force generated at the

interface.

RF-MEMS devices can fail due to the delamination of bonded thin film materials. Bond

failure of dissimilar materials and similar metals in such a wafer-to-wafer bonding can

cause delamination also. Dampening is also critical for RF-MEMS due to the

mechanical nature of the parts and resonating frequency. It is mainly due to the

atmospheric gases. Hence, RF-MEMS devices should be properly sealed. Since, RF-

MEMS devices have mechanical moving parts; they are more susceptible to

environmental degradation.

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Thermal and heat transfer issues [12] become more complicated by packaging various

functional components into a tight space. Heat dissipation from the packaged system

to environment becomes significant. The miniaturization also increases such

problems. In a thin package, heat spreads in surrounding electronic components and

Microsystems.

5. CONCLUSION

The research of the degradation mechanisms and failure modes of RF-MEMS devices

and the physics behind them is very challenging. This paper presented a brief report

of the interesting complex degradation and failure mechanisms concerning to the

reliability of novel RF and microwave devices which are fabricated using micro-machining technology. The RF-MEMS technology is facing various problems related to

reliability, stability and lifetime estimation. Although our knowledge about reliability

issues is still incomplete, yet this paper is presented to create interest about this field

in future. Contact material reliability, fabrication related issues, electrostatic actuation

related and packaging related reliability problems are reported here. Two main

factors for the life time durability of RF-MEMS devices (ohmic RF-MEMS switch

devices and capacitive switches) were identified as ohmic contact reliability and

charge distribution within the dielectric. Packaging plays a key role in ensuring the

long-term reliability of RF-MEMS devices.

REFERENCES

[1] G. Rebeiz, RF MEMS in full CMOS radio SOC, in 2005 MTT-S Int. MicrowaveSymp. Workshop Notes, WSE: Full CMOS Radio, Long Beach, CA, June 12-17,

2005.

[2] Hartzell et al.,MEMS Reliability, Characterization and Test, Proc. SPIE, Vol.4558.

[3] J. L. Zunino III, et Al., Micro-electromechanical Systems (MEMS) ReliabilityAssessment Program for Department of Defense Activities, Proc. 2005 NSTI

Nanotechnology Conference and Trade Show (Nanotech), Anaheim, CA, May

2005, Vol. 3, pp.463-466.[4] J. Maciel, Recent Reliability Results in RF MEMS, Proceedings of the 2005 IEEE

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MTTS Int. Microwave Symp. Workshop Notes, WFE: Recent Applications in RF

MEMS, Long Beach, CA, June 12-17, 2005.

[5] J. Schimkat,Contact materials for micro relays, Proc. of the 11th AnnualInternational Workshop on MEMS, 1998, pp.190-194.

[6] J. Wibbeler, et Al., Parasitic charging of dielectric surfaces in capacitivemicroelectro- mechanical systems (MEMS), Sensors and Actuators A: Physical,

pp.74-80, Nov. 1998.

[7] R. Maboudin,Anti-Stiction Coatings for surface micromachines, SPIE Vol. 3511,pp. 108-113, 1998.

[8] S. Arney, Gasparyen, and H. Shea, Designing MEMS for Reliability, short coursegiven at SPIE Photonics West, Jan. 2001.

[9] S. Arney,Designing for MEMS Reliability, MRS Bulletin, 2001, pp.296.[10] W. Nakayama,Thermal issues in micro systems packaging, IEEE Transactions

on Advanced Packaging 23(4), pp.602-607, 2000.

[11] X. Rottenberg, et Al., Distributed dielectric charging and its impact on RF MEMSdevices, in Proceeding of 34th European Microwave Conference, Amsterdam,

Oct 2004, Vol. 1, pp.77-80.

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ISSN: 2231-5608

Vol.1, Issue 6

Aditya Informatics & Research Centre

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Engineering College BharatpurBharatpur, Rajasthan, India

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Lachoo Memo. College of Sci. and Tech.

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Reliance Communication

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